Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity Under the 2012 and 2013 Plans

Stock option activity under the Company’s 2012 Equity Incentive Plan (the “2012 Plan”) and 2013 Equity Incentive Plan (the “2013 Plan”) for the nine months ended September 30, 2018 is summarized as follows:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2017		2,158,348		$	4.40		8.1	$	436
Granted		1,530,150			2.38
Exercised		(919	)		2.56
Cancelled		(32,001	)		2.71
Outstanding at September 30, 2018		3,655,578		$	3.57		8.3	$	—
Vested and expected to vest at September 30, 2018		3,288,917		$	3.69		8.2	$	—
Exercisable at September 30, 2018		1,460,898		$	4.86		7.2	$	—

Summary of Stock-Based Compensation for all Options Granted and Restricted Stock Awards

The allocation of stock-based compensation for all options granted and restricted stock awards is as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
	(in thousands)				(in thousands)
Research and development	$	131	$	86	$	352	$	326
General and administrative		307		283		897		907
Total stock-based compensation expense	$	438	$	369	$	1,249	$	1,233

Summary of Weighted-Average Assumptions Used in the Black-Scholes Option Pricing Model to Determine the Fair Value of the Employee Stock Option Grants

The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Risk-free interest rate		2.92	%		1.95	%		2.77	%		2.03	%
Expected volatility		86.4	%		60.0	%		87.5	%		63.1	%
Expected term (in years)		6.08		6.08				6.08			6.08
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%

Summary of Weighted-Average Assumptions Used in the Black-Scholes Option Pricing Model to Determine the Fair Value of the Non-Employee Stock Option Grants

The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the non-employee stock option grants were as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Risk-free interest rate		1.97	%		1.35	%		1.97	%		1.28	%
Expected volatility		74.0	%		59.7	%		74.0	%		63.0	%
Expected term (in years)		6.08			6.08			6.08			6.08
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%